The epitaxial growth of Group III-V nitride semiconductor materials on a substrate can be effected by molecular beam epitaxy (MBE) or by metal organic vapor phase epitaxy (MOVPE).
MOVPE takes place in a apparatus which is commonly at atmospheric pressure but sometimes at a slightly reduced pressure of typically 10 kPa. Ammonia and species providing one or more Group III-V elements to be used in epitaxial growth are supplied substantially parallel to the surface of the substrate upon which epitaxial growth is to take place, thus forming a boundary layer adjacent to, and flowing across, the substrate surface. It is in this gaseous boundary layer that decomposition to form nitrogen and the elements to be epitaxially deposited takes place so that the epitaxial growth is driven by gas phase equilibria.
MBE takes place in an apparatus in which the substrate is held within an ultra-high vacuum environment, typically about 10.sup.-3 Pa at a relatively low substrate temperature. The nitrogen precursor is supplied to the vacuum chamber fitted by means of a supply conduit and species providing one or more Group III-V elements are supplied from appropriate sources within heated effusion cells filed with controllable shutters. The nitrogen precursor and species supplied from the effusion cells travel across the vacuum chamber and reach the substrate where epitaxial growth takes place in a manner which is driven by the deposition kinetics.
In both MBE and MOVPE growth of gallium nitride (GaN) layers, the lack of a suitable substrate that is thermally matched and lattice-matched to GaN has necessitated the use of intermediate thin GaN or AlN buffer layers grown at low temperature on the substrate before growth of the GaN epilayer. Such a buffer layer is required to enable laying down of a GaN layer of sufficiently high quality for the fabrication of an optoelectronic device made from III-V nitrides, as disclosed in, for example, U.S. Pat. Nos. 5290393 and 5,385,862. The quality of the epilayer, and hence of the device itself, is highly sensitive to the nature of the buffer layer. However, difficulties can be encountered in the control of the growth of such buffer layers resulting in significant run-to-run variability of the layer.
In the case of the growth of a GaN buffer layer by MBE, the active nitrogen necessary for epitaxial growth may be generated either by a plasma source or by decomposition of ammonia (NH.sub.3) at the substrate surface. Conventionally, growth of the buffer layer is effected at a temperature of around 250 to 450.degree. C. which is less than the optimum temperature for GaN growth (which is 750 to 850.degree. C.). It has been found necessary to use such low growth temperatures for the buffer layer in order to reduce the surface diffusion length of impinging atoms on the growth surfaces and thereby promote uniform and laminar nucleation of the buffer layer across the substrate surface.
In the case of a MBE growth method in which the active nitrogen is obtained by decomposition of ammonia at the growth surface, however, the decomposition rate of ammonia at the growth surface rapidly decreases at temperatures of less than 550.degree. C., as shown by M. Kamp et al. in Proceeding of Topical Workshop on III-V Nitrides, Nagoya, Japan (1995). This makes the formation of such a buffer layer very difficult using such a source of nitrogen.
However, in the case of MBE using ammonia as the nitrogen precursor, the surface diffusion length of impinging atoms on the growth surface is dependent not only on the growth temperature but also on the arrival rate of nitrogen atoms at the growth surface. Generally, the surface diffusion length is inversely proportional to the square root of the atom flux or growth rate. In a method disclosed by Granjean et al. in Appl. Phys. Lett. 64, 2664 (1994), an increase in the length of time taken for lattice-mismatched InGaAs to nucleate in a laminar manner on a GaAs substrate using elevated growth rates, as opposed to the time taken using more moderate growth rates, has been demonstrated. In this method, a buffer layer of the same material as the substrate is grown at the optimum temperature for GaAs growth, that is about 580.degree. C., prior to growth of the InGaAs layer in order to bury the substrate surface which may have some inhomogeneities. However, the provision of such a buffer layer may not be necessary if the substrate surface is of sufficient quality.
However, high growth rates of the order of 2.mu.m/hr or more have not previously been obtainable for MBE growth of III-V nitrides. This has been due to restrictions on the amount of atomic nitrogen which can be supplied to the growth surface caused by inefficient plasma sources of nitrogen, low pumping speeds and large source-to-source distances. In the particular case of MBE growth using ammonia as a nitrogen precursor, it has been demonstrated by M. Kamp et al. in Proceedings of Topical Workshop on III-V Nitrides, Nagoya, Japan (1995) that the amount of active nitrogen generated by ammonia decomposition is dependent on the ammonia flux supplied only at substrate temperatures above 600.degree. C. However, it has not proved possible to increase the ammonia flux sufficiently to achieve growth rates much greater than about 1.2 .mu.m/hr.